Open the Firefox disk image (DMG) file and drag the Firefox program to the application folder on your hard drive.
You shouldn't double-click the Firefox application to run it from the disk image, but drag it out of the DMG folder instead.

The OP is on a Mac, so try this instead:
Make sure that Firefox gets installed properly.
Open the Firefox disk image (DMG) file and drag the Firefox program to the application folder on your hard drive.
You shouldn't double-click the Firefox application to run it from the disk image, but drag it out of the DMG folder instead.
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*https://support.mozilla.org/kb/Installing+Firefox+on+Mac
*http://kb.mozillazine.org/Installing_Firefox#Mac_OS_X